This invention relates to the field of thermal spray technologies for applying coatings, and in particular to detonation thermal spray.
At this time, detonation spray technology is mainly used to apply coatings to workpieces exposed to severe wear, heat or corrosion and is fundamentally based on using the kinetic energy produced in the detonation of a combustible mixture of gases to deposit powdered coating materials on workpieces.
Coating materials typically used in detonation processes include powder forms of metals, metal-ceramics and ceramics and are applied to improve resistance to wear, erosion, corrosion, as thermal insulators and as electrical insulators or conductors.
Spraying by detonation is performed by spray guns which basically consist of a tubular detonation chamber, with one end closed and another open, to the latter being attached a tubular barrel. A combustion gas mixture is injected into the detonation chamber and ignition of the gas mixture is achieved with a spark plug, causing a detonation and consequently a shock or pressure wave which travels at supersonic speeds inside the chamber and then inside the barrel until it leaves through the open end of the barrel.
The coating material powder is generally injected into the barrel in front of the propagating shock wave front and is then carried out of the open end of the barrel and deposited onto a substrate or workpiece placed in front of the barrel. The impact of the coating powder onto the substrate produces a high-density coating with good adhesive characteristics.
This process is repeated cyclically until the part is adequately covered.
Powder feeders commercially available supply a continuous feeding which makes them adequate for high-velocity or plasma spray technologies. Since detonation is a discontinuous process, however, it requires discontinuous powder feeding.
Feeders used in detonation devices provide discontinuous feeding by using devices which control the amount of powder supplied to the detonation barrel in each explosion. These devices, however, are designed specifically for each type of gun, that is, they cannot be interchanged for use with other guns or in other machines which require feeding powder.
With respect to the powder measuring system employed. they can be classified in two categories:
a) Mechanical: These devices use moving mechanisms (valves, spindles, gears, etc.) to introduce constant quantities of powder in each detonation cycle. Devices of this type are described for example in U.S. Pat. NO. 3,109,680, and in European Patent 0 484 533.
These devices have the main advantage of providing precise measurements but are however of great complexity they have many components. Their reliability is low since they require periodic maintenance to maintain the precision of the measurement. Finally their productivity is low since they is are limited to low operation frequencies.
b) Pneumnatic: These devices use gas pulses synchronised with the detonation pulses to introduce the powder cyclically in the detonation barrel, with these pulses sometimes being obtained from the detonation process itself. The elegance and mechanical simplicity of these devices has contributed to their wide use despite their precision being questioned. There are also numerous Patent documents such as PCT/US96/20129 by the same authors.
These devices share the characteristic of incorporating a volume or deposit in which a limited amount of powder is stored, which, by gravity, feeds another volume or dosage chamber which feeds the detonation barrel by a gas impulse. The disadvantage of these systems is their lack of precision in the amount of powder dosed, mainly due to their difficulty, over long spray periods, of keeping stable the volume and/or pressure of the feeding deposit. This is due to the fact that part of the detonation wave enters the powder feeding deposit, pressurizing it so that the powder falls under gravity and due to the pressure existing in the deposit at each time.
In addition, since the amount of powder entering the dosage chamber cannot be perfectly controlled, the degree of fluidization produced by the impulse gas cannot be controlled either, and thus it is difficult to know precisely the amount of powder injected into the barrel.
Furthermore, since in these devices feeding from the deposit to the dosing chamber is by gravity, when the detonation gun generally handled by an industrial robot) assumes positions in which the powder deposit is not vertical, the powder will not fall into the dosage chamber continuously. Thus, it is difficult to ensure a constant feeding.
Document GB-A-2 192 815 is known in prior art, which describes a detonation coating device comprising a barrel open at one end, a gas feeding system, a blast initiating assembly and a powder bath metering unit consisting of a vertically oriented bunker changing at its lower part into a vertical tube under which, inside the barrel, a horizontal rack is located. The barrel is oriented vertically with its axis parallel to the axis of the bunker, whereas the tube is connected to the barrel through the closed butt-end of the latter.
In British Patent GB-2192815, the deposit containing the powder to be discharged is placed vertical, with the powder falling on the distribution tray under the action of gravity, which moans that the gun can only operate in positions where the deposit, the distribution duct and the tray are arranged vertically, as otherwise the powder would not be supplied. Thus, this gun cannot be used mounted on a robot arm as the latter""s motion would be limited by the position of the powder deposit.
The powder is fed from a closed deposit, so that as the deposit is emptied, conditions inside it change, particularly the temperature and pressure. Thus it is not possible to ensure a control of the amount of powder introduced.
The dosing of the powder to be used in each blast cycle is determined by size and arrangement of he distribution tray, and is interrupted when the powder reaches a height in the tray which obstructs the outlet of the distribution duct, so that the gas carries the amount of powder present in the tray. Thus, there is not an exact control of the dosed amount as the tray may be more or less filled depending on the chamber conditions and on the powder.
The powder feeder is on a fixed position on the rear wall of the combustion chamber, so that it is only suitable for performing certain types of coatings. This is so because, depending on the type of coating dust employed, a specific barrel length is required, and as the dust feeder is on the rear wall the length of the gun will always be the same. Thus, for coatings which require different barrel lengths we would need a different gun, suitable for this coating. The gun of GB-2192815 is therefore quite inflexible as regards the coatings which may be obtained.
This detonation coating device is not suitable for providing good coatings with any kind of materials, but it is only appropriate for particular coatings.
The present invention fully solves the above disadvantages by using an injection system which allows employing a conventional type continuous powder feeder for feeding a detonation spray system, the powder injection being performed cyclically, in synchronization with the gun spray frequency and with great precision in the powder dosage.
The system proposed allows directly connecting the gun and the continuous powder feeder and consists of a dosage chamber which receives the continuous powder feeding and a conduit which directly communicates the chamber with the gun barrel. Consequently, in each detonation cycle, the detonation pressure wave reaches the dosage chamber, momentarily interrupting the feeding so that the ensuing suction of the detonation wave carries the powder contained in the dosage chamber,thereby injecting it into the gun barrel.
With this object the dosage chamber communicates with the gun barrel by a direct tubular conduit of small diameter, so that the pressure wave that advances through the barrel passes to the communication conduit and on reaching the dosage chamber undergoes a sudden expansion which fills the chamber with pressurized gas, blocking the entry of the powder feeding conduit. In this way, the feeding of powder from the continuous feeder is cyclically interrupted, and it is therefore possible to determine the exact amount of powder present in the dosage chamber at the time of detonation.
The sudden expansion of the gas in the dosage chamber creates a turbulence which produces the fluidization of all the powder contained in the dosage chamber so that the suction process, which follows the detonation, carries all the powder contained in the chamber, so that it is possible to control the exact amount of powder injected into the barrel. In addition, as the pressure wave is composed of hot gases produced in the combustion process the interaction of these gases with the powder contained in the dosage chamber produces a preheating of the powder which favors its fluidization.
In this way, when the pressure wave generated in the detonation passes the communication conduit of the dosage chamber, the low pressure generated after the detonation wave creates a suction which carries the gas contained in the dosage chamber and the fluidized powder. The powder carried reaches the barrel, where it remains until the pressure wave generated in the following detonation cycle carries it, depositing it on the surface of the part to be coated.
With this injection system the pressure wave from the detonation is made to perform the injection of powder into the barrel cyclically and synchronized with the gun firing frequency, thus transforming a continuous powder feeding into a pulsed injection to the gun barrel without using complex mechanical devices.
In addition, the expansion created by the dosage chamber reduces the velocity of the pressure wave preventing it from eroding the dosage chamber and advancing into the powder feeder, eliminating the risk of it producing irreparable damages to the feeding system.
The dosage chamber presents an elongation or auxiliary chamber opposite the communication conduit to the detonation barrel which is meant to increase the length of the dosage chamber to reduce the force of the impact and therefore the effects of the erosion produced by the encounter of the gases and the powder in this area of the dosage chamber.
The device of the invention presents the following advantages:
It favors a cyclical interruption of the feeding by the detonation pressure wave.
It favors a preheating and fluidization of the powder by its interaction with the hot gases of the combustion.
It allows feeding a precise amount of powder in each explosion by the suction effect which follows the pressure wave in each detonation.